Due to their extraordinarily good ratio of stiffness or strength to density, core composites, i.e. composite sandwich structures having a lightweight core, have a broad range of application especially in the field of aircraft construction.
Well-known conventional core composites are generally formed of an upper and a lower cover layer or cover ply, between which is located, for example, a honeycomb-type core structure formed of vertically extending cells with a hexagonal cross section, for increasing the stiffness of the resulting composite sandwich structure.
For example metallic corrosion-protected aluminum foils, or non-metallic materials, such as Nomex®- or Kevlar®/N636-paper for example, are used for forming the core structure. Both the Nomex®-paper as well as the Kevlar®/N636-paper are coated with phenolic resin in a submersion process for increasing the mechanical strength thereof.
The local stiffening or reinforcement of a core structure as represented by the above described true honeycomb structure is not problematic. Due to the small-volume repeat units represented by the individual hexagonal cells extending perpendicular to the cover layers, a reinforcement material can be filled, pressed, foamed, poured or cast locally into the intended reinforcement zone, and is confined within the pertinent cells without spreading uncontrollably throughout a larger area of the interior of the core. The stiffening material may, for example, be a curable synthetic plastic material, for example in a synthetic resin and/or a synthetic plastic foam. In order to enable a connection of the core composite to other components, the core composite can be bored in the area of the local reinforcement zone, so that securing elements or fixtures can be securely mounted to the bored hole of the reinforcement zone.
If necessary, through the use of relevant known methods, a securing or mounting element, for example an insert nut or some other insert, can be introduced into the core composite. Alternatively, the securing element can also be introduced without previous reinforcement of the core composite according to relevant known methods.
In contrast to the above described true honeycomb-type core composites, in which a spatial limitation is always provided by the relatively small, individual cells extending perpendicular to the cover layers, new types of core composites, especially formed of three-dimensional folded comb or cell structures, comprise an open or drainable structure. Namely, such pleated or folded core structures include fold or pleat valleys that form open channels extending continuously in the plane of the composite structure, i.e. along or parallel to the cover layers from edge-to-edge of the composite structure. Thus, the core channels of such a core composite extend open and unobstructed through large areas (or even along the entire length or width) of the finished composite sandwich. Thereby, for example, it is possible to guide various types of lines (e.g. electrical lines, hydraulic lines, water lines, air lines, etc.) through the core composite without previously having to cut or machine a passage therethrough while impairing the mechanical properties of the core composite.
If, for the reinforcement of the above described drainable core structures, a pasty or viscous hardenable material is introduced into the core, depending on the viscosity thereof; it can more or less uncontrolledly spread out throughout each affected core channel of the open core structure, and thus laterally unbounded throughout large areas of the composite sandwich.
Similar considerations pertain for bored holes introduced into the finished core composite. In contrast to true honeycomb structures, in which fundamentally only a limited number of structural units are opened with a bored hole, and thus introduced substances are always spatially limited within the affected cells, viscous or pasty substances can spread out uncontrollably over larger distances in open drainable core structures.
Therefore the standard methods according to the state of the art are not usable for producing spatially limited reinforcements and for integrating securing elements in open drainable core structures.